If I understand, the following should do the trick
/ stimulustimes = [0 = fixation; 500 = stimulus; 600 = blank]
/ timeout = 1500
Fixation, stimulus, and blank are all separate stimulus elements defined in your script. Fixation is the fixation point, stimulus is the word/nonword, and blank is just a blank text to erase the stimulus.
The timeout command indicates that the trial should advance if the subject doesn't respond in 1500 ms.
Hope this helps,